present invention relates to a continuous process for producing yarn of polyethylene terephthalate polyester and, more particularly, to an improved process for producing high denier high stress industrial yarn by controlling variations of intrinsic viscosity of the molten polymer. However, other polymers produced by similar processes, for example, polycondensation reactions, copolyesters, and the like, could utilize the concepts disclosed herein.
Throughout this disclosure, the term "intrinsic viscosity" shall be taken to be a measure, within limits, of the sought after quality of the polymer being produced. The melt-spinnable polyester for use in the present process prior to extrusion is selected to have an intrinsic viscosity (IV) of about 0.5 to 2.0 deciliters per gram, and preferably a relatively high intrinsic viscosity of 0.8 to 1.0 deciliters per gram, and most preferably 0.85 to 0.94 deciliters per gram. The IV of the melt-spinnable polyester may be conveniently determined by the equation ##EQU1## where .eta.r is the "relative viscosity" obtained by dividing the viscosity of a dilute solution of the polymer by the viscosity of the solvent employed (e.g. orthochlorophenol) measured at the same temperature, and c is the polymer concentration in the solution expressed in grams/100 ml. The starting polymer additionally commonly exhibits a degree of polymerization of about 140 to 420, and preferably of about 140 to 180. The polyethylene terephthalate starting material commonly exhibits a glass transition temperature of about 75.degree. to 80.degree. C. and a melting point of about 250.degree. to 265.degree. C., e.g. about 260.degree. C.
Also, it should be recognized that there is a direct relationship between intrinsic viscosity (IV) and molecular weight of the polymer being produced. This relationship is clearly shown as follows:
IV=1.7.times.10.sup.-3 .times.(no. avg. mol. wt.).sup.0.83
where the parenthetical phrase "no. avg. mol. wt.", that is, "number average molecular weight" means ##EQU2## where m.sub.i is the molecular weight of molecule n.sub.i or the total weight of molecules divided by the number of molecules. Hence, the higher the molecular weight, within limits, the better the quality of the resulting product.
The present invention is principally concerned with improving known processes of producing high stress spun yarns for industrial purposes. Industrial yarns are heavy duty yarns and are used, for example, in the formation of tire cord, conveyor belts, seat belts, V-belts, hosing, sewing thread, carpets, and the like, and have properties of strength and tenacity which greatly exceed those characteristics of textile yarns used for clothing and normal household furnishings. Known processes relating to the production of such high stress spun yarns are disclosed in the U.S. Pat. No. 4,101,525 to Davis et al issued July 18, 1978 and U.S. Pat. No. 4,195,052 to Davis et al issued Mar. 25, 1980, as well as McClary U.S. Pat. No. 4,414,169 issued Nov. 8, 1983. As noted in those patents, a polymer having an IV of about 0.35 to 0.70 deciliters per gram is commonly selected when forming textile yarns and a polymer having an IV of about 0.7 to 1.0 deciliters per gram is commonly selected when forming industrial yarns.
It has long been known that there is a close correlation between control of IV and production of a homogeneous polymer having good uniform quality. It is for such a purpose that the U.S. Pat. Nos. 3,878,379 and 4,106,098, to Moody, Jr. et al, U.S. Pat. No. 4,153,766, to Koide and U.S. Pat. No. 4,237,261 to Kawamura et al, all provide viscometers between each of the various stages or reactors of a cascade system for continuously producing polyethylene terephthalate polyester. In each instance, the viscometers are arranged to provide feedback and/or feedforward signals to adjacent reactors. Control is achieved by making adjustments to set points of operating parameters in each reactor of set points based on a variety of factors, including pressure, agitator speed, inlet and outlet melt viscosity, inlet and outlet temperatures, internal temperature of a reactor, flow rate, and pump speed. The Moody, Jr. et al and Koide et al patents claim to hold variations of IV to within +/-0.005 IV units.
According to the U.S. Pat. No. 3,493,345, to Windley, changes in the amount of power consumed by a motor driving a mechanical pump or mixing blade during the production process are detected and necessary adjustments made accordingly to control viscosity. Control is similarly disclosed in U.S. Pat. No. 3,502,622, to Reichel et al, according to which melt viscosity of the preliminary condensation product is continuously measured. In that patent disclosure, if there is excessive variation of the melt viscosity, alcohol or other solvent is added to the production system through a controlled metering device.
Of particular note is the process disclosed in the patent to U.S. Pat. No. 3,503,937 to Allen et al. The Allen et al process utilizes a pair of viscometers, one having a probe positioned in the pipeline near the outlet from the separation vessel and a second having a probe positioned downstream of the first probe and near the spinning unit. When the viscosity at the second probe differs from a desired viscosity level manually established at a controller, a signal is transmitted to the controller to adjust the preestablished level by altering the vacuum within the vessel. The Allen et al patent recites a relative viscosity of the polymer so produced as being within 0.25 relative viscosity units of the desired level 95 percent of the time.